The human respiratory system is characterised by the phenomenon called expiratory flow limitation, that is by the fact that during expiration the flow of gases expelled depends on the expiratory pressures only up to a certain threshold value, beyond which the flow cannot be increased even if expiratory pressures are increased. The threshold value is function of the lung volume (being higher at high volumes then decreasing with the reduction of the lung volume) and is, in healthy subjects, considerably higher than the maximum flows obtained during spontaneous breathing. The difference between maximum expiratory flow and expiratory flow during spontaneous breathing measured at the same lung volume constitutes the reserve of expiratory flow. Such reserve is used in situations such as physical exercise, in which the metabolic requirements and, consequently, the lung ventilation increase. The resulting increase of the tidal volume and of the respiratory frequency therefore requires the increase of both inspiratory and expiratory flows. Several obstructive pathologies of the respiratory system (chronic obstructive pulmonary disease or COPD, asthma, . . . ) modify the mechanical properties of airways and lung parenchyma, notably reducing the reserve values of expiratory flow until it is annulled. In addition, the reduction can be so marked that it forces the patient to breathe at higher lung volumes than usual to make use of the dependence of the maximal flow on lung volume and to obtain the expiratory flows needed for a correct gas exchange. This phenomenon is knows as “dynamic hyperinflation”. Breathing at higher lung volumes, however, means both using the inspiratory muscles in a geometric condition which is unfavourable to them (reduced muscular length and consequent reduction of the capacity to produce force) and having the thoracic wall expanding to volumes at which it is more stiff. All these effects combine to determine a significant increase in the respiratory work, excessively tiring the respiratory muscles and thus limiting the capacity of the patient to carry out even simple physical activity. To know whether a patient is flow-limited during spontaneous breathing is therefore of fundamental importance for the diagnosis and definition of the rehabilitation and pharmacological therapy, as well as the assessment of their effectiveness.
As of today the methods used for identifying whether a subject is limited in the expiratory flow can be divided into two main categories: those based on the measurement of the oesophageal pressure and those that do not use this measurement. The former are currently considered the only reliable ones, but they require a latex balloon connected to a catheter-pressure transducer system for measuring the oesophageal pressure to be inserted into the lower third of the oesophagus. Esophageal pressure is considered an excellent estimate of the pleural pressure and therefore it allows the estimation of alveolar pressure. From the relationship between alveolar pressure and flow at the airway opening it is possible to assess the presence of expiratory flow limitation during quiet breathing in different ways (for example studying the so called “isovolume pressure flow curves” or analysing Mead and Whittenberger graphs). Unfortunately this procedure is invasive and it is not tolerated by a large percentage of subjects, in addition it requires a lot of time (both for the insertion of the balloon and for the patient to adapt to it) and it may significantly alter the breathing pattern of the subject.
The methods belonging to the second category are based on the determination of the flow-volume curves carried out in body plethysmograph and on the application of negative pressure to the airways during the expiratory phase or on abdominal compression. However these methods are characterised by both theoretical and practical limitations and therefore they are usually considered not very reliable and, in the first case, require the collaboration of the subject.
A study by M. Vassiliou, R. Peslin, C. Saunier, C. Duvivier, entitled “Expiratory flow limitation during mechanical ventilation detected by the forced oscillation method”, European respiratory Journal, 1996, suggests a method for determining expiratory flow limitations based on the forced oscillations technique able of indicating the possible presence of expiratory flow limitation induced by bronco constrictor drugs (methacholine) in rabbits submitted to mechanical ventilation. In this study it can be observed that expiratory flow limitation is responsible for systematic variations of the imaginary part (Xrs) of the total respiratory input impedance (Zrs) in mechanical models of a single collapsible airway and in mechanically ventilated rabbits. The Applicant believes that the analyses described in the article mentioned above were carried out under extremely critical and particular conditions, that is on mechanically ventilated animals in which the limitation of the expiratory flow was induced pharmacologically and on mechanical models that simulate the behaviour of only one single airway and that therefore they do not reproduce real conditions occurring in humans with obstructive diseases (hundreds of thousands of airways placed in series and in parallel in the tracheal-bronchial tree constricted in a very heterogeneous pattern).